The meaning of 是 is "to be". For example: 我是…… I am... 你是…… You are... 他是…… He is... The reason it sometimes means "yes" is because questions can be answered with the verb used in the question. If you ask 你是……吗？, the verb used is 是 so you can answer in the positive 是 (yes) or negative 不是 (no). [不 goes before the verb] So 不是中文 means not-is-Chinese, meaning "It isn't Chinese".

In addition to meaning "yes," 是 can also be translated as "to be" (as in, "am, is, are"). So 不是中文means "(it) is not Chinese." Unfortunately, 不中文is ungrammatical. :(

不 doesn't mean anything on its own. It's used in front of a verb to negate it. 是 is a verb that means "to be". So 不是 means "is not“. Ex. 我是澳州人。 - I'm Australian. 我不是中国人。 - I'm not Chinese. There is no exact equivalent of "yes" in Chinese. The positive response to a question is to repeat the verb, and the negative response is to negate it. If the verb is 是, then 是 does mean 'yes' and 不是 does mean no. If the verb is different, then you need to use that verb. Ex. 你是学生吗? This means "Are you a student?". 是 is the verb here, so 是 means 'yes' and 不是 means ‘no'. 你说汉语吗？ This means "Do you speak Chinese?“. 说 is the verb, so 说 means 'yes' and 不说 means 'no'.

It's quite simple. 是 Means be. 不是 means be not.

the word 是 doesn't mean "yes" it means "to be" 不 是 中文 = isn't written Chinese language